Prior to the present invention, it was generally known that aluminosilicate materials, for example, mullite, a naturally occurring, high temperature performance material composed essentially of chemically combined aluminum, oxygen and silicon atoms x(Al.sub.2 O.sub.3).SiO.sub.2, where x is 1.5 to 2.0, was an attractive candidate for a variety of high temperature applications. In addition, mullite, unlike closely related aluminum-oxygen-silicon materials, is known to be highly resistant to attack by strong acids and other corrosive reagents, as taught by R. F. Davis and T. A. Pask, "Mullite", pp. 37-76 in "High Temperature Oxides", Part IV, Allen M. Alper, ed., Academic Press, NY (1971). However, no technique was known for making mullite in a form, such as a high temperature, corrosion resistant, coating. It would be highly desirable, therefore, to provide a procedure whereby mullite could be made synthetically in an appropriate form to utilize its outstanding properties.
As shown by W. A. D. C. Technical Report 58-160 ASTIA document No. 155675, The Air Force Inorganic Polymer Program, R. L. Rau, (June 1958), Pages 21-25, silicon-oxygen-aluminum polymers can be made by effecting reaction between an aluminum chelate dialkoxide, for example, diisopropoxyaluminum acetylacetonate and a difunctional saline, such as dimethylacetoxysilane. Reaction was carried out in boiling toluene to produce a variety of products varying from soft resins, waxes, or powders. I have found that the aforementioned aluminum-oxygen-silicon materials of R. L. Rau provide glass-like coatings when heated at temperatures exceeding 350.degree. C. in an oxidizing atmosphere, for example air. However, the resulting aluminosilicate coatings fall outside of the mullite composition range, and do not provide optimum coating characteristics on ceramic or metal substrates in particular applications. Additional procedures for making organoaluminosilanes are shown by S. N. Barisov et al, Organosilicon Heteropolymers and Heterocompounds, Plenum Press, New York (1970), however, none of these procedures lead to the preparation of organoaluminosilanes with Al/Si atomic ratios in the range appropriate for mullite.
H. Dislich, New Routes to Multicomponent Oxide Glasses, Angewandte Chemie, International Edition, Vol. 10, pages 383-434 (1971) has described the preparation of coherent multicomponent oxide glass coatings on various substrates using mixtures of metal alkoxides in organic solvents. Similarly, Yoldas and Partlow, Formation of Continuous Beta Alumina Films and Coatings at Low Temperatures, Ceramic Bulletin, Vol. 59, No. 6, (1980) pages 640-642, describe the preparation of continuous films of NaAl.sub.11 O.sub.17 on ceramic substrates using solutions of the corresponding metal alkoxides. In both reports, removal of the organic component is effected by hydrolysis of the organometallic film after deposition and the resultant metal oxide films do not possess the desired thermal and chemical stability characteristic of mullite.
As shown by K. S. Mazdiyasni et al, Synthesis and Mechanical Properties of Stoichiometric Aluminum Silicate (Mullite), Pages 548-552, Vol. 55, No. 11, Journal of the American Ceramic Society, a method for preparing mullite is provided by reacting aluminum trisisopropoxide and silicon tetrakisisopropoxide under reflux conditions in isopropyl alcohol. The resulting alkoxide solution can be ammoniated to produce the corresponding hydroxy aluminosilicate which can be dried in vacuum to produce mullite powders. However, the aforementioned technique was unsuitable for applying a mullite coating onto various substrates.